1. Field of the Invention
The invention relates to the field of systems for installing heat generating circuit board modules into enclosures, and, in particular, to a system for clamping the circuit board modules to guide rails in the enclosure providing good thermal conduct therebetween.
In electrical equipment on-board aircraft, low weight and compactness are critical parameters in the design thereof. In order to more easily repair and replace individual electronic components, most of them are mounted on easily removable circuit board modules. The circuit board modules are rack mounted in enclosures and, typically, are automatically plugged into mother boards on the back wall upon insertion via mating connector halves mounted on both the motherboard and module. The weight and size limitations require that they be closely packed in the enclosures and, since a great many of these components generate heat when in operation, both air and liquid cooling systems have been used. One such method is to clamp the circuit board modules to the mounting guide rails in the enclosure and use the guide rails to transfer and distribute the heat to a solid metal guide rail support plate. Cooling fluid is pumped over the back side of the plate absorbing the heat, and thus, transferring the heat out of the enclosure. Alternately, the cooling fluid can be pumped directly through internal passages in the guide rails providing better heat transfer efficiency. However, regardless of the cooling system used, efficient heat transfer depends upon obtaining a good thermal contact (clamping) between the edge of the circuit board module and the guide rails.
Present day thermal clamps are mechanical in nature. For example, U.S. Pat. No. 4,480,287 "Module Retainer Apparatus" by K. Jensen discloses a clamp having three or more interlocking wedged shaped bodies which are slotted and keyed together to allow tightening using a single screw which can be turned from the module insertion side of the enclosure. A center body of the clamp includes a U-shaped channel with a square cross-section and sloped ends. Holes are provided for attaching to either the module or the enclosure guide rails. Each end body has a keying guide extending from a sloped end. The screw extends through a clearance hole in an end-body, then through the center body U-shaped channel and, finally, into a threaded hole in opposite end body. When the screw is tightened, the end body slides along the ends of the center body causing them to wedge the module against the enclosure guide rails. The end bodies are held in alignment with respect to the center body using keying guides which extend from the end of the bodies into slots in the base of the center body. This clamp is designed to provide both locking of the module within the enclosure as well as thermal clamping, thus, depending upon friction to maintain the circuit board module within the enclosure. Additionally, such "wedge body" type thermal clamps have numerous components which are subject to wear and jamming.
U.S. Pat. No. 4,318,157 "Apparatus for Mounting Circuit Cards" by W. J. Rank et al. discloses a somewhat similar design; however, the screw is also adapted to readily engage a hole in the back wall of the enclosure providing positive locking of the circuit board module within the enclosure as well as thermal clamping. While this design provides positive locking it is also composed of numerous parts which are subject to wear and jamming. Furthermore, rotation of a screw to provide thermal clamping as well as locking of the module within the enclosure is a time consuming operation. In U.S. Pat. No. 4,414,605 "Positive Locking Mechanism" by J. J. Chino et al. a screw is used to secure the circuit board module in the enclosure while a spring biased over center latch arrangement is used to operate a three-part wedge body assembly to achieve thermal clamping. While the use of the over center lever or latch provides rapid thermal clamping, however, the mechanism is complicated and the use of a separate screw to lock the module within the enclosure increases installation time and requires the use of a tool.
U.S. Pat. No. 4,751,963 "Thermal Conductance Retainer for Electronic Printed Circuit Boards and the Like" by T. T. Bui et al. discloses a more complicated version of the wedge body clamp concept. Here a single wedge body assembly is mounted in the enclosure and moves in a direction parallel to the circuit board module; however, the opposite surface of the wedge body is tapered and rides in a groove tapered parallel to the guide rail. When the screw is tightened, the wedge body is forced into the tapered groove and, thus against the edge of the circuit board module. Thus, this design only allows for the mounting of the clamp to the enclosure. Thus, if the clamp became inoperative it would have to be removed from the enclosure or the whole electronic enclosure would have to be removed from the vehicle. This is a major draw back when compared to thermal clamps which are mountable to the circuit board module. U.S. Pat. No. 4,298,904 "Electronic Conduction Cooling Clamp" by P. C. Koenig discloses another variation on the wedge body type clamp. Here compound angles on the wedges translate an axial force into two forces at right angles thereto and result in the wedge bodies contacting multiple heat transfer surfaces. Again, as with all the wedge body type clamps, they are subject to wear and jamming.
In U.S. Pat. No. 4,157,583 "Circuit Board Clamping Assembly" by F. J. Basmajian et al. a clamp is disclosed wherein a shaft is rotatably mounted along the edge of the circuit board module and incorporates a cam at its center. Rotation of the cam causes the cam to contact a guide rail forcing the edge of the circuit board module in contact with the opposite guide rail. While this device is simple in nature, it does not provide for a continuous clamping force to be applied along the edge of the circuit board module and also depends upon frictional forces to lock the module within the enclosure or the use of a separate locking system. U.S. Pat. No. 3,992,653 "Zero insertion Force Card Guide" by P. H. Richardson et al. and U.S. Pat. No. 4,550,836 "Clamp Arrangements" by S. M. Ellis both disclose lever actuated spring lock devices, which upon locking, force springs under compression load into engagement with the edges of the circuit board modules. However, the use of the springs do not provide a positive positional lock and, under certain circumstances, high vibration loads could allow the circuit board modules to vibrate. Finally, U.S. Pat. No. 4,779,674 "Sawtooth Card Retainer" by C. T. McNulty discloses a circuit board module thermal clamp which comprises a mated pair of sawtooth bars which slidably engagable teeth. The bars are laterally spread apart and held by a screw or spring, in order to push against and hold the edge to a guide rail. Here the drawback is that the device is subject to jamming by foreign debris which may lodge in the sawtooth teeth.
Thus, it is a primary object of the subject invention to provide a thermal clamp for electronic circuit modules mounted within electronic enclosures.
It is another object of the subject invention to provide a thermal clamp for electronic circuit board modules mounted within an electronic enclosure combined with a mechanism for locking the module within the enclosure.
It is another object of the subject invention to provide a thermal clamp for electronic circuit board modules mounted within an electronic enclosure combined with a mechanism for locking the module within the enclosure which does not require tools.
It is a further object of the subject invention to provide a thermal clamp for circuit board modules mounted within electronic enclosures utilizing a minimum number of parts.
It is a still further object of the subject invention to provide a thermal clamp for electronic circuit board modules mounted within electronic enclosure utilizing hydraulic pressure to provide the thermal clamping force.